An infrared detector called the “bolometer,” now well known in the art, operates on the principle that the electrical resistance of the bolometer material changes with respect to the bolometer temperature, which in turn changes in response to the quantity of absorbed incident infrared radiation. These characteristics can be exploited to measure incident infrared radiation on the bolometer by sensing the resulting change in its resistance. When used as an infrared detector, the bolometer is generally thermally isolated from its supporting substrate or surroundings to allow the absorbed incident infrared radiation to generate a temperature change in the bolometer material, and be less affected by substrate temperature.
Modern microbolometer structures were developed by the Honeywell Corporation. By way of background, certain prior art detectors and/or arrays, for example those manufactured by Honeywell, Inc., are described in U.S. Pat. Nos. 5,286,976, and 5,300,915, and 5,021,663, each of which is hereby incorporated by reference. Such detectors include those microbolometer detectors that have a two-level microbridge configuration: an upper level and a lower level form a cavity that sensitizes the bolometer to radiation of a particular range of wavelengths; the upper level forms a “microbridge” that includes a thermal sensing element; the lower level includes read-out integrated circuitry, and reflective material to form the cavity; the upper microbridge is supported by legs which thermally isolate the upper level from the lower level and which communicate electrical information therein to the integrated circuitry.
A list of references related to the aforesaid structure may be found in U.S. Pat. No. 5,420,419. The aforesaid patent describes a two-dimensional array of closely spaced microbolometer detectors which are typically fabricated on a monolithic silicon substrate or integrated circuit. Commonly, each of these microbolometer detectors are fabricated on the substrate by way of a commonly referred to “bridge” structure that includes a temperature sensitive resistive element that is substantially thermally isolated from the substrate. This aforesaid microbolometer detector structure is herein referred to as a “thermally-isolated microbolometer.” The resistive element may, for example, be comprised of vanadium oxide material that absorbs infrared radiation. The constructed bridge structure provides good thermal isolation between the resistive element of each microbolometer detector and the silicon substrate. An exemplary microbolometer structure may dimensionally be on the order of approximately 50 microns by 50 microns.
In contrast, a microbolometer detector that is fabricated directly on the substrate, without the bridge structure, is herein referred to as a “thermally shorted microbolometer,” since the temperature of the substrate and/or package will directly affect it. Alternately, it may be regarded as a “heat sunk” microbolometer since it is shorted to the substrate.
Microbolometer detector arrays may be used to sense a focal plane of incident radiation (typically infrared). Each microbolometer detector of an array may absorb any radiation incident thereon, resulting in a corresponding change in its temperature, which results in a corresponding change in its resistance. With each microbolometer functioning as a pixel, a two-dimensional image or picture representation of the incident infrared radiation may be generated by translating the changes in resistance of each microbolometer into a time-multiplexed electrical signal that can be displayed on a monitor or stored in a computer. As used herein, the term “pixel” is equivalent to the term “microbolometer”. The circuitry used to perform this translation is commonly known as the Read Out Integrated Circuit (ROIC), and is commonly fabricated as an integrated circuit on a silicon substrate. The microbolometer array may then be fabricated on top of the ROIC. The combination of the ROIC and microbolometer array is commonly known as a microbolometer infrared Focal Plane Array (FPA). Microbolometer focal plane arrays that contain as many as 640×480 detectors have been demonstrated.
Individual microbolometer pixels will have non-uniform responses to uniform incident infrared radiation, even when they are manufactured as part of a single microbolometer FPA. This is due to small variations in the detectors' electrical and thermal properties as a result of the manufacturing process. This non-uniformity in the individual microbolometer response characteristics, commonly referred to as spatial non-uniformity, must be corrected to produce an electrical signal with adequate signal-to-noise ratio for image processing and display. The characteristics contributing to spatial non-uniformity, among others, include the infrared radiation absorption coefficient, resistance, temperature coefficient of resistance (TCR), heat capacity, and thermal conductivity of the individual detectors.
The magnitude of the response non-uniformity can be substantially larger than the magnitude of the actual response due to the incident infrared radiation. The resulting ROIC output signal is difficult to process, as it requires system interface electronics having a very high dynamic range. In order to achieve an output signal dominated by the level of incident infrared radiation, processing to correct for detector non-uniformity is required.
Methods for implementing an ROIC for microbolometer arrays have used an architecture wherein the resistance of each microbolometer is sensed by applying a uniform electric signal source, e.g., voltage or current sources, and a resistive load to the microbolometer element. The current resulting from the applied voltage is integrated over time by an amplifier to produce an output voltage level proportional to the value of the integrated current. The output voltage is then multiplexed to the signal acquisition system.
Gain and offset corrections are applied to the output signal to correct for the errors that may arise from the microbolometer property non-uniformities. This process is commonly referred to as two-point correction. In this technique two correction coefficients are applied to the sampled signal of each element. The gain correction is implemented by multiplying the output voltage by a unique gain coefficient. The offset correction is implemented by adding a unique offset coefficient to the output voltage. Both analog and digital techniques have been utilized to perform this two-point non-uniformity correction.
The current state-of-the-art in microbolometer array ROICs suffers from two principal problems. The first problem is that the larger arrays increase substrate temperature. A second problem is that a larger microbolometer introduces non-uniformities in the ROIC integrated circuit output signal thereby requiring a large instantaneous dynamic range in the sensor interface electronics that increases the cost and complexity of the system. Current advanced ROIC architectures, known in the art, incorporate part of the correction on the ROIC integrated circuit to minimize the instantaneous dynamic range requirements at the acquisition systems interface.
A technique for minimizing the effect of substrate temperature variations is to provide substrate temperature stabilization so as to maintain a substantially constant substrate temperature. One common technique employed for substrate temperature stabilization is the use of what is commonly referred to as “thermoelectric cooling.” As used herein, the term “thermoelectric cooler” is equivalent to the term “thermoelectric stabilizer”—both of which are commonly used in the art and refer to an apparatus and technique for raising and lowering the temperature of a substrate to maintain the substrate at a substantially constant temperature.
An unstabilized (i.e. no temperature stabilization of the substrate) microbolometer focal plane array is taught in U.S. Pat. No. 5,756,999, entitled, “Methods and Circuitry for Correcting Temperature-Induced Errors in Microbolometer Focal Plane Array”, issued to Parrish, et. al. Therein, a bias correction method and a pre-bias correction method are taught. With regard to the bias correction method, as stated therein, “According to the bias-correction method, a unique bias amplitude is applied to each detector during the integration period to support uniformity correction. The bias-correction method can be implemented as an adjustable voltage, current, or load bias that is applied to the microbolometer detectors during the integration (measurement) period . . . . The bias-correction value is applied during the integration period of the microbolometer detector using an adjustable voltage source . . . . The bias-correction value is controlled by the output of a digital-to-analog converter (DAC) . . . . The adjustable bolometer bias may be used to correct the optical gain of the signal for uniform output at a particular substrate temperature in conjunction with single-point offset correction . . . to remove residual fixed offsets.”
Although the aforesaid patent sets forth improved methods for correcting temperature induced errors in microbolometer focal-plane arrays, these methods still remain complex and time consuming.
Another method for controlling bias for a microbolometer focal plane array permitting an unstabilized (i.e. no temperature stabilization of the substrate) microbolometer focal plane array is taught in U.S. Pat. No. 6,444,983, entitled, “Microbolometer Focal Plane Array With Controlled Bias,” in the name of T. McManus, et. al., and is incorporated herein by reference thereto. As taught therein, a single thermally-shorted microbolometer detector is employed for controlling the magnitude of an electric signal bias source that is applied to all microbolometers of a focal plane array on a substrate. A calibration bias source magnitude is determined and continually adjusted as a function of the reading value of the resistance of the thermally-shorted microbolometer at calibration, and readings of the thermally-shorted microbolometer after each image sample is taken. With the bias controlled in a manner as just described, temperature induced errors are minimized thereby permitting the microbolometer focal plane array to be employed without the need of any thermoelectric stabilization of the substrate.
Another source of error in microbolometer focal plane arrays is the variation in “responsivity” of each microbolometer with changes in temperature. This is because each microbolometer is a temperature sensitive resistor having a sensitivity that varies due to minute variances in the manufacturing process from one microbolometer to another. The temperature of a microbolometer is affected by the bias current flowing there through, since that current inherently warms the microbolometer due to the power dissipated. Heat is also transferred to the microbolometer through the focal plane array's substrate. Heat is also transferred to the microbolometer by infrared radiation focused on the microbolometer.
In order to produce a high quality image from a microbolometer focal plane array, the differences in responsivity from one microbolometer to another need to be taken into account, and the change in microbolometer resistance due to infrared radiation impinging thereon should be separated from any resistance changes due to all other sources of heat transfer.